1. Field of the Invention
This invention relates to a process for preparing substituted phthalic anhydrides. More particularly, it relates to a process for preparing 4-methylphthalic anhydride.
2. Related Art
Phthalic anhydrides are valuable raw materials for making various useful products. These anhydrides are useful as intermediates in the chemical synthesis of herbicides and particularly in the synthesis of certain herbicides used to protect cereal crops. Other uses for these raw materials include polycyclic dyes, alkyd and epoxy resins, polyesters and plasticizers.
Various processes are known for preparing substituted phthalic anhydrides. In two such processes 4-methyl-1,2,3,6-tetrahydrophthalic anhydride is dehydrogenated either by sulfur or by bromine in acetic acid. Yields of 4-methylphthalic anhydride of 59 percent to 87 percent are claimed for the former method, while the latter gives only a 16 percent yield [see Izv. Akad. Nauk. SSSR, Ser. Khim., 6, 1315 (1973) English translation, page 1271 and D. Craig, Journal of American Chemical Society, Vol. 72, page 3732, (1950)].
U.S. Pat. No. 2,391,226 (Clifford et al., Dec. 18, 1945) discloses addition products of chlormaleic anhydride and dichlormaleic anhydride prepared by the Diels-Alder reaction and the dehydrochlorination of these products in the presence of a catalyst, such as a secondary or tertiary amine. However, the six carbon ring is usually only partially dehydrogenated yielding a substituted dihydrophthalic anhydride.
U.S. Pat. No. 2,264,429 (Bergman, Dec. 2, 1941) discloses a process for preparing substituted phthalic anhydride in a single reaction. This reaction involves the combination of the condensation reaction between a diene and maleic anhydride and the dehydrogenation reaction. This combination is achieved by carrying out the condensation reaction in nitrobenzene or another nitrated aromatic substance, which not only acts as a diluent, but also as a dehydrogenating agent by reducing itself and giving the corresponding amine.
U.S. Pat. No. 4,302,396 (Tsujimoto et al., Nov. 24, 1981) discloses the preparation of 4-methylphthalic anhydride by the dehydrogenation of tetrahydro-4-methylphthalic anhydride with sulfur.